1. Field of the Invention
The present invention relates to a method and apparatuses for radio communication preferably used in a communication system which employs the HSDPA (High Speed Downlink Packet Access) method, one of mobile communication systems.
2. Description of the Related Art
In the present 3GPP (3rd Generation Partnership Project), W-CDMA (Wideband-Code Division Multiple Access) method, one of the third-generation mobile communication systems, is being standardized. One of the standardization objects is to define HSDPA which provides a downlink transmission speed of 14 Mbps at maximum.
HSDPA adopts an adaptive coding and modulation method, in which communication between a base station and a mobile station is switched between, for example, QPSK (Quadrature Phase Shift Keying) modulation method and 16QAM (Quadrature Amplitude Modulation) method according to a radio environment between these stations.
Further, HSDPA employs an HARQ (Hybrid Automatic Repeat reQuest) method which is characterized by that, if a mobile station detects an error in received data from a base station, the base station retransmits the same data responsive to a request from the mobile station and the mobile station performs an error correction decoding process using both received data and the retransmitted data. In HSDPA, efficient use of received data even with an error in the above manner makes it possible to enhance the gain of error correction and thereby reduce the number of retransmission processes.
HSDPA mainly uses radio channels of HS-SCCH (High Speed-Shared Control Channel), HS-DSCH (High Speed-Downlink Shared Channel), and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
HS-SCCH and HS-DSCH are common downlink channels (i.e., in the direction from a base station to a mobile station). HS-SCCH is a control channel to transmit various parameters (L1 information) concerning data transmitted through HS-DSCH. The parameters are exemplified by modulation type information indicating that data is transmitted through HS-DSCH in which modulation method, the number of spreading codes to be allocated (a code multiplication number), a process number of HS-DSCH, a retransmission/new indicator whether or not transmission data is retransmission data or new data, and a pattern used for rate matching to be performed on transmission data.
HS-SCCH is capable of concurrently transmitting a control signal to a number of mobile stations using a number (e.g., four) of spreading codes, and each of the mobile stations specifies HS-SCCH destined for the own mobile station with reference to UE-ID (User Equipment-IDentity).
HS-DPCCH is a dedicated control channel to uplink communication from an individual mobile station to a base station, and is specifically used to transmit an ACK signal (ACK information) and a NACK signal (NACK information), respectively indicating successfully reception and failure reception of data through HS-DSCH, from the mobile station to the base station. In the event of failure in data reception, if the data received is a CRC (Cyclic Redundancy Check) error, the mobile station transmits a NACK signal to the base station, which performs retransmission control.
Additionally, HS-DPCCH is used to send a base station a CQI (Channel Quality Indicator) that is a measurement result (e.g., SIR (Signal to Interference Ratio)) of receipt quality of a signal from the base station when measurement is performed by a mobile station. The base station judges, according to the CQI, whether or not the downlink radio environment is good, and switches a modulation method to a method able to transmit data in a higher speed if the judgment result is positive and conversely switches to a method to transmit data in a lower speed if the judgment result is negative (in other words, performs an adaptive modulation). Here, a frame format for HS-DPCCH is shown in FIG. 21 and a field for HS-DPCCH is shown in FIG. 22.
As described above, in a system employing HSDPA, data transmission between a base station and a mobile station is confirmed, and if a loss of a PDU (Packet Data Unit) or a transmission-and-reception error is detected, the system carries out FEC (Forward Error Correction) and ARQ (Automatic Repeat reQuest).
Further, in order to enhance the communication efficiency by efficient operation of HARQ, there has been proposed an N-channel Stop and Wait method in which N (an integer of two or more) HARQ processes are provided in a radio communication between a base station and mobile stations. In this method, before reception confirmation information (ACK/NACK information) concerning a certain transmission process reaches either station (the mobile or base station) from the opposite station (the base or the mobile station), the first station can transmit a PDU for the next process to the opposite station.
For example, in an N-channel stop-and-wait method where N=6, when a PDU with process number “1” is transmitted from the base station to a mobile station, the base station sequentially transmits PDUs for six processes at maximum in advance before ACK/NACK information for the PDU of process number “1” from the mobile station reaches the base station. That improves communication efficiency in a radio communication system even large in response delay. In HARQ employing the N-channel stop-and-wait method, a number (N) of processes are independent one after another and reception confirmation information (ACK/NACK information) concerning each process is associated with the individual process and transmitted to the opposite station.
However, data pieces concerning one or more of a number of processes do not correctly reach the opposite station for various reasons such as deterioration in communication state.
As a result, if a result of recognition of ACK/NACK information at the transmitter side (the mobile station) does not coincide with that at the receiver side (the base station), an abnormal operation exemplified by the below (1) to (4) occurs. FIG. 23 is a diagram showing examples of a transmission operation performed in the base station when the base station has misrecognized ACK/NACK information from the mobile station.
(1) If the base station misrecognized ACK information from the mobile station to be NACK information, the base station judges that the mobile station (the opposite station) has not correctly received the data and retransmits the same data to the mobile station (a retransmission process).
(2) If the base station misrecognized ACK information from the mobile station to be DTX (not reached), the base station judges that the mobile station does not recognize the transmission of the data to the mobile station and transmits the same data as new data.
(3) If the base station misrecognized NACK information from the mobile station to be ACK information, the base station judges that the base station has normally received the data and transmits next new data.
(4) If the base station misrecognized NACK information from the mobile station to be DTX information, the base station judges that the mobile station does not recognize the transmission of the data the mobile station and transmits the same data as new data. In this case, if IR (Incremental Redundancy) combination is activated, again of IR combination cannot be obtained because the base station transmits the data without changing the RV (Redundancy Version) (Chase combination).
As described above, if the base station misrecognizes ACK/NACK information, the base station does not retransmit data which should be retransmitted or unnecessarily retransmits data that has been normally received by the mobile station.
As a solution to the above problem, Non-Patent Reference 1 discloses that previously-known training patterns called Preamble and Postamble (hereinafter called PRE and POST, respectively) are attached to ACK/NACK information so as to sandwich the ACK/NACK information as shown in FIG. 24, so that adjustment of a detection threshold value by such training patterns can improve accuracy of detection of ACK/NACK information at the receiver side, avoiding an increase in the transmission electricity power.
Further, the following Patent Reference 1 discloses the technique that soft-decision combination values of all the symbols of ACK/NACK information are combined and ACK/NACK information is judged on the basis of a comparison of the combination result and the predetermined threshold value.
The following Non-Patent Reference 2 describes an allowable error rate on a radio propagation path between a mobile station and a base station.    [Patent Reference 1] Japanese Patent Application Laid-Open (KOKAI) No. 2005-51713    [Non-Patent Reference 1] TR25.899V6.1.0 (September 2004) (6.7 ACK/NACK Transmit Power Reduction for HS-DPCCH with preamble and postamble)    [Non-Patent Reference 2] TS25.104 V7.4.0 (June 2006) (8.10 Performance of ACK/NACK detection for HS-DPCCH)
The above-described art disclose contrivances to reduce the possibility of misrecognition occurrence on ACK/NACK information.
However, considering the case where the control channel (HS-DPCCH) through which the above receipt confirmation information (ACK/NACK information) is transmitted is not demodulated in the receiver station (i.e., the base station), these techniques cannot ensure normal (sure) transmission of ACK/NACK information.
In particular, an allowable error rate of a radio propagation path between a mobile station and a base station in a predetermined environment is defined to be less than the specification condition 10−2 (see above Non-Patent Reference 2). This allowable error rate is for a long period and is therefore excessively large for an instant error rate in communication on the high-speed movement or in a multi-path fading environment, resulting in tendency of occurring an error.
As understood from the above description, since misrecognition of ACK/NACK information is unneglectable, the problem is that normal data communication (retransmission) is not carried out for a process ACK/NACK information of which has been misrecognized.
Here, description will be made in relation to data (packet) communication (a retransmission process) performed when ACK/NACK information has been misrecognized with reference to accompanying drawing FIG. 25, which is a flow diagram explaining data communication in the above case (3), for example.
In a communication system employing an N-channel stop-and-wait method, the base station transmits new data (a packet) having process number “1” (Process #1) and packet number “X” (Packet No. X) to the mobile station (packet transmission). If the mobile station cannot normally receive the data piece in question, the mobile station transmits NACK information to the base station (feedback of the reception result of the packet).
The base station receives the reception confirmation result (the feedback information) from the mobile station and, if however misrecognizes the NACK information to be ACK information due to an error occurring at the radio propagation path, transmits new data having process number “1” (Process #1) and packet number “X+1” (Packet No. X+1) when performing a transmission process of the next process umber “1” (HARQ block).
The mobile station plans to perform a combination of retransmission process on the data of process number “1” and packet number “X”, but cannot carry out the planed process because the new data received from the base station has process number “1” and packet number “X+1”. Further, the base station does not retransmit data of process number “1” and packet number “X” to the mobile station, the data (packet) is lost.
As described above, if the base station misrecognizes ACK/NACK information from the mobile station, normal operation for data communication cannot be performed. As another example, if the base station misjudged an ACK response to be NACK or DTX, the base station should transmit a new data but actually carries out a retransmission process whereby the communication rate is lowered.
As described above, the process ACK/NACK information of which is misrecognized is temporarily deadlocked until the next reception confirmation information (i.e., ACK/NACK information) of the same process is correctly received by the base station.
Accordingly, in an HARQ process performed in N-channel stop-and-wait method where N=6, deadlocking of one process lowers communication rate (throughput) by as high as 17 percent (one sixth).